Cellulosic biomass is the most abundant renewable natural resource. Generated at a rate of ˜100 billion dry tons/year by the biosphere, cellulosic biomass has the potential to replace the world's demand for diminishing fossil fuels. However, according to Zhang, Y. H. P. “One of the most important and difficult technological challenges is to overcome the recalcitrance of natural lignocellulosic materials, which must be enzymatically hydrolyzed to produce fermentable sugars.” See, (Zhang, Y. H. P., et al., “Outlook for cellulase improvement: Screening and selection strategies.” Biotechnol. Adv., 2006, 24: 452-481).
Cellulose is a polysaccharide consisting of 100 to 20,000β-1-4 linked glucose units. Cellulases, the class of enzymes that hydrolyze cellulose, have attracted immense interest for their ability to degrade cellulosic biomass into glucose for biofuel production. Three cellulase sub-classes (endoglucanase, exoglucanase, β-glucosidase) work synergistically to hydrolyze cellulose. Endoglucanases hydrolyze intramolecular β-1-4-glucosidic bonds in insoluble cellulosic material to produce new chain ends. Exoglucanases progressively hydrolyzed the chain ends liberating small water-soluble oligosaccharide products. The soluble products are finally hydrolyzed by β-glucosidase into glucose (Schulein, M., “Protein engineering of cellulases.” Biochim. Biophys. Acta-Protein Struct. Molec. Enzym., 2000, 1543(2): 239-252).